In general, a fuel cell is an electricity generating system directly converting chemical reaction energy into electric energy through an electrochemical reaction between the oxygen in air and the hydrogen contained in a hydrocarbon fuel such as methanol, ethanol, or natural gas.
A polymer electrolyte membrane fuel cell (PEMFC), recently developed, has excellent output characteristics, low operation temperatures, and fast starting and response characteristics. A basic PEMFC includes a stack, a fuel tank, and a fuel pump supplying the fuel from the fuel tank to the stack. The stack is the main body of the fuel cell. The PEMFC may further include a reformer reforming the hydrocarbon fuel to generate hydrogen to be supplied to the stack.
During operation of the PEMFC, the fuel stored in the fuel tank is supplied to the reformer by a fuel pump. The reformer reforms the fuel and generates hydrogen. The stack generates electric energy through an electrochemical reaction between the hydrogen and the oxygen. Heat is also generated as a byproduct of this process.
In the above fuel cell system, the stack generating electric energy is constructed with several to tens of unit cells each having a membrane-electrode assembly (MEA) and a separator. A separator is also referred to as a bipolar plate in the art.
The MEA has an anode and a cathode attached on both surfaces of an electrolyte membrane. The separator serves as a passage through which hydrogen and oxygen needed for reactions of the fuel cell are supplied to the anodes and cathodes on the electrolyte membrane. In addition, the separator serves as a conductor serially coupling the anodes and cathodes of the MEA.
Through the separator, the hydrogen-containing fuel is supplied to the anode, and oxygen or oxygen-containing air is supplied to the cathode. During the process, electrochemical oxidation of fuel occurs at the anode, and electrochemical reduction of oxygen occurs at the cathode, causing a flow of electrons. Electricity, heat, and water can be obtained from the electron flow.
The stack must be maintained at a proper operating temperature in order to secure stability of the electrolyte membrane and to prevent deterioration in performance of the electrolyte membrane. For this reason, the stack has cooling channels. A low temperature coolant such as water or air flowing through the cooling channels can cool the heated stack. Cooling depends on how much the cooling channels can contact the MEA of each unit cell and carry its heat away by conduction. The measure of contact between the cooling channels and the unit cell is contact area of the cooling channels per unit area of the unit cell.
In the conventional fuel cell system, there is a temperature difference between central and circumferential regions of the unit cell in the stack, because the circumferential areas may be cooled by adjacent air. Yet, the contact area of the cooling channel per unit area of the unit cell remains the same among different regions of the unit cell. Because the temperature difference between different regions of the unit cell is not addressed by the cooling system, the cooling efficiency of the stack decreases and the stack performance deteriorates.